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Recently, my partner gave a great idea for utilising my old Kindle: generate a “newspaper” each morning from a bunch of RSS feeds, and email it to the Kindle using “Send-to-Kindle” feature (a blog post about this project is in the works).

I loved this idea, and thought it would be no problem to get a Python script up and running periodically on my Raspberry Pi home server using cron. However, I ran into various issues along the way (some of which were not so easy to resolve), so I’m collating all the configuration changes I made in the hopes that it will be useful to someone one day. You can find the full repo for this project here, and I have also included my Dockerfile, docker-compose.yml and crontab at the end of this TIL.

1. Double check the user

A lot of problems with cron come down to user privileges. Each user has their own crontab, and then there is the system-wide root crontab. The first issue I ran into with creating a cron job inside a container was that Docker created the crontab as a non-root user. This issue presented itself to me when I tried to run the following command, to list the current cronjobs in the Docker container:

docker-compose exec container-name crontab -l

This returned the following output:

no crontab for root

Now, it is not necessarily a problem to have non-root cron jobs, but just make absolutely certain that you are creating the jobs with the user you expect. For me, I wanted to run as root, so I added to following line to my docker-compose.yml:

user: root

Now, the root user will be used when building your Docker image and the created crontab will be where you expect.

2. Missing dependencies

When cron calls your Python script, you may run into issues with ModuleNotFoundError or ImportError, where Python cannot find your installed packages. This is because cron does not have access to your system environment variables, including the Python path. You can resolve most of these errors with imports by adding the PYTHONPATH environment variable to your crontab. This should be the path to your site-packages folder, something like this:

PYTHONPATH=/usr/bin/local/python3

You may also need to add a shebang (#!) to your Python script to direct cron to the correct version. You can find the Python location with one of the following commands:

which python
which py
which python3

NOTE: These commands must be performed in your Docker container when it is up and running. In docker-compose syntax this would be the following (with the name of your container instead of container-name):

docker-compose exec container-name which python3

You can then add this to the top of your Python script, as follows:

#!/usr/bin/local/python3

3. Still missing dependencies

Some modules will still run into errors even when the PYTHONPATH variable has been set. In particular, I ran into problems with reportlab and Pillow/PIL:

ImportError: cannot import name '_imaging' from 'PIL'

This was solved by adding the system PATH to the crontab as well. The system path is included in the default crontab that is created when you first run crontab -e:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Therefore, it is a good idea to include it if you are making a new crontab to make sure cron can find everything it needs to.

4. Check relative paths in Python

By default, cron runs from the default root path. Therefore, both your call to Python in your crontab and the filepaths within Python should either be relative to root (i.e /main.py rather than main.py) or just use full paths instead.

5. “Failed to build wheel” and related errors

This error is related to Python inside a Docker container rather than cron. However, someone might still find it useful. When you install your requirements.txt, you may encounter errors such as

legacy-install-failure
error: command '/usr/bin/gcc' failed with exit code 1
fatal error: Python.h: No such file or directory

I was able to resolve these by adding python3-dev, wheel and Cmake to my requirements.txt. These are sometimes required when packages include other binaries or need to compile other code when installed.

6. Other useful tips

• crontab.guru is a great resource for checking cron syntax
• Installing vim/nano in your Docker container to make the debugging stage easier. This is especially useful for changing your crontab to run much more frequently, or adding debugging messages etc., when the container is up.

I hope this helped you resolve some errors! I’ve included my Dockerfile, docker-compose.yml and crontab below if you want to set up a similar project or adjust your own files. The full repo is also available here.

Dockerfile:

FROM python:3

COPY . .
RUN python3.11 -m pip install --no-cache-dir -r requirements.txt

RUN touch /var/log/cron.log

RUN apt-get update \  
&& apt-get install cron -y

RUN chmod +x main.py

RUN crontab crontab 

CMD cron -f

docker-compose.yml:

version: "2.4"

services:
  watchman:
    platform: "linux/arm64/v8"
    image: watchman:latest
    container_name: watchman
    restart: always
    user: root
    build:
      context: build
      dockerfile: Dockerfile

crontab:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PYTHONPATH=/usr/bin/local/python3
15 7 * * * python3 /main.py >> /var/log/cron.log 2>&1